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The present invention relates to the manufacture lithium polymer secondary batteries, and more particularly to a method for manufacturing a lithium polymer secondary battery by wrapping anode and cathode plates by an isolation film while rotating a cell initially formed during an assembling process in a state in which the isolation film is tensioned by a desired tension.
Recently, portable appliances such as notebook PCs and cellular phones have been widely distributed. Such a wide distribution of portable appliances has resulted a greatly increased demand for rechargeable secondary batteries of a high capacity and a high performance. A rechargeable lithium ion secondary battery has been proposed which includes a cathode made using, as an active substance, a carbonic material capable of occluding and releasing lithium ions, an anode made of lithium based mixed oxides capable of performing charging and discharging of lithium ions in accordance with a structural deformation thereof, and a nonaqueous electrolyte serving as a medium for lithium ions. This lithium ion secondary battery is being widely used. However, this ion battery has problems associated with stability in that it exhibits a high reactivity with moisture, and uses an excessive amount of thermally unstable electrolyte. Furthermore, it requires use of a metal can as its battery package. For this reason, there is a degradation in energy density, and a limitation on battery shape.
In order to eliminate such problems or drawbacks, a lithium polymer secondary battery using polymer electrolyte has been proposed. This lithium polymer secondary battery exhibits reduced leakage of electrolyte while allowing free design of the battery""s shape. Also, there are advantages of a miniature, light, and thin structure, and a superior stability. Examples of such a lithium polymer secondary battery are disclosed in U.S. Pat. No. 6,468,693, and other patents.
FIG. 1 illustrates a method for manufacturing a lithium polymer secondary battery, using a winding process. This method is also adopted in U.S. Pat. No. 6,468,693. The lithium polymer secondary battery according to this patent is of a jelly roll type. That is, the lithium polymer secondary battery has a structure in which a jelly roll is inserted into a can which is, in turn, capped and then sealed in accordance with a laser fusion process. The cathode and anode of this lithium polymer secondary battery are fabricated by coating a polymer binder, conductive powder, and an active electrode material on both surfaces of a copper thin plate and an aluminum thin plate, respectively. In order to achieve an attachment of electrode taps, the cathode and anode have non-coated portions. The non-coated portions of the cathode and anode are electrode taps made of nickel and aluminum materials, respectively. The two electrodes are connected to external terminals of the battery via the electrode taps, respectively. One of the electrode taps attached to the non-coated electrode portions is attached to the bottom or side surface of the can when the jelly roll is inserted into the can, whereas the other tap is attached to the cap. Such an assembling method and structure provides certain advantages. That is, the anode and cathode are in uniform contact with each other by virtue of a tension applied to an isolation film during a winding process while being physically pressed against the wall of the can. Accordingly, when the battery performs a charging or discharging operation in accordance with an electrochemical reaction, the entire electrode surface is uniformly used. Thus, the battery basically has a superior performance while exhibiting a high performance even for lengthy charge and discharge cycles. There is another advantage given by virtue of the wall of the cell being made of a metal material having a high mechanical strength. That is, the jelly roll is filled in the interior of the can in a strongly pressed state in accordance with the physical pressing force present between the jelly roll and the can wall. It is also possible to minimize the deformation of the can in a thickness direction caused by an internal deformation force generated from the jelly roll. Thus, the density of energy per unit volume of the finally manufactured battery can be increased. In addition, it is possible to prevent the material contained in the battery from being outwardly leaked, and to prevent external foreign matters from penetrating into the interior of the battery.
However, the lithium polymer secondary cell manufactured using the winding process has a wound shape which does not correspond to a perfectly planar shape, but corresponds to a deformed oval shape. For this reason, it is impossible to fit the jelly roll in the battery without formation of gaps. Accordingly, there is a difference between the stress concentrated on the edge of the cell and the stress concentrated on the central portion of the cell. As a result, there is a problem in that the life of the battery is reduced.
On the other hand, U.S. Pat. No. 4,048,397 discloses a zig-zag stacking method in which a plurality of electrodes are stacked within a certain area. The cell manufactured in accordance with this method has a planar stacked structure, so that there is no cause for any stress difference. In accordance with this method, however, a gap between each electrode plate and the isolation film is inevitably formed during the stacking process, as shown in FIG. 2. Due to such a gap formed between each electrode plate and the isolation film, the battery may be swelled up at its edge after prolonged charging and discharging operations, so that the life of the battery is reduced.
Therefore, the present invention has been made in view of the problems involved with the above mentioned winding method and zig-zag stacking method, and an object of the invention is to provide a method for manufacturing a lithium polymer secondary battery, which is capable of minimizing the gap between each electrode and an isolation film, and uniformly distributing stress over the entire surface of each electrode, thereby extending the life of the rechargeable battery.
In accordance with the present invention, this object is accomplished by providing a method for manufacturing a lithium polymer secondary battery including a plurality of cells each including an anode plate, a cathode plate, and an isolation film adapted to insulate the plates from each other, comprising the steps of: preparing a plurality of anode plates each having a desired size, a plurality of cathode plates each having a desired size, and an isolation film having a desired size for the cells; wrapping one of the anode plates by the isolation film; wrapping one of the cathode plates by the isolation film in a state in which it is overlapped with the anode plate, thereby forming one of the cells; and stacking the remaining anode plates and cathode plates at both sides of the formed cell while rotating the formed cell, and simultaneously wrapping them by the isolation film so that t-hey are alternately arranged in a state in which the isolation film is tensioned by a desired tension.